1. Field of the Invention
This invention relates to an LED video display apparatus and method of operating the same and more particularly to such and apparatus with an increased wide angle viewing area and a method of normalizing the output to an individual color LED or LEDs.
2. Description of the Prior Art
Large signs and billboards have been in wide use for many years as a medium for advertising and for imparting information to the public. Traditionally, signs and billboards have been used to exhibit a single advertising theme, product, or message. Due to the fixed print nature of this medium, it did not lend itself to displaying a larger series of ideas as would be common with a medium such as television.
Within the last several decades, this has changed. The advances of technology in such areas as light emitting sources, and in particular light emitting diodes (LEDs), has made it possible to provide a series of messages to signs and billboards. As a result, a wide variation of words and images can be projected in a variety of ways from the same sign to advertise and inform the public.
For example, during sporting events animated signs can heighten a fan""s involvement in the game with slogans and images. Also, advertisers can get the attention of viewers to encourage purchasing products ranging from such things as a specific beverage, to items that may not be available at the game. The fixed print surface portion of billboards can be replaced with multiple light sources that can exhibit multiple advertisers and messages to passing motorists. Applying LEDs to this medium has increased the effectiveness of the presentation and has captured public notice.
LED Video/Graphic Boards, as they are commonly known, have been in production and use throughout Asia for nearly ten years. Here, color LEDs are arranged in pixels and the pixels form an array. Various numbers of arrays can be combined to produce a display apparatus. Such display have uses in applications such as billboards and signs.
Only within the last five years has it become economically feasible to produce Large-Area Full-Color Video/Graphic Boards. These units have appeared in such places as Times Square, Las Vegas, Rock Concerts, Theme Parks, and Trade shows. However, the current use of LEDs for these purposes has significant drawbacks.
Initially LEDs suffered from substantial technical limitations. They proved expensive, had low power output, produced a substantial amount of heat as a by-product in operation and due to the accompanying electronic circuitry, exhibited poor reliability in sign and billboard applications, also such LEDs did not provide good resolution or contrast. Improvements over the last several years have resulted in lower cost for certain types of LEDs while exhibiting higher power output and increased reliability. This, along with advances in electronic power circuitry, allowing for lower heat loss, has resulted in increased use of LEDs in billboard and sign applications. However, a major problem still remains in that the displays do not provide a very good level of contrast or resolution and viewability from a, variety of viewing angles.
The typical LED radiates light in substantially a circular cross sectional pattern or in a conical three-dimensional volume. Usually the greatest power distribution of the light is directly in front of the LED and the power drops off dramatically at the edges of the LED radiating cone. Usually this variation in power over viewing angle is characterized as a normal or Gaussian distribution depending on the effective focal length of the integral LED collecting lens. If a viewer moves from directly in front of the LED, there is a substantial amount of light that is not directed toward the viewer. As the viewer moves to the side of the LED, the viewer receives substantially less light. Not only is this inefficient, allowing for a very limited optimal viewing point, but the wasted light can interfere with the contrast of other LEDs in a display system. One way to overcome this problem is to use an LED that can produce light in a fixed pattern that has a more useful distribution of power over a wide angle.
The Nichia Corporation produces a xe2x80x9csuper ovalxe2x80x9d LED that distributes its light more evenly over a wider horizontal viewing plane, i.e. over a range of approximately xe2x88x9250 degrees from the center and +50 degrees from the center across the horizontal plane. However, between 50 and 90 degrees on both sides of the zero point of the plane the luminosity decreases in nearly an exponential fashion. This translates into a viewing range that is better than a standard LED system, but provides a harsh transition for the viewer beyond approximately 50 degrees from the center of radiation in both directions. Furthermore, the super oval LED accomplishes distributing the light by building into the LED a special optical element. Such a process naturally increases the cost of the LED. Finally, since the super oval type LED has the collecting lens cast within the LED as well as a reflector, the lens and reflector cannot be removed and changed if conditions so warrant.
A characteristic of super oval type LEDs is that the form of the output beam is not well controlled. When dispersion occurs along the horizontal axis there is a dispersive effect on the vertical axis. Thus, a dispersion in the horizontal makes for a dispersion in the vertical whether or not a vertical dispersion is desired.
A further problem with existing apparatus is that the power projected by an individual LED, pixel, and pixel array apparatus may vary. This is due to the variable output of individual LEDs which may not be consistent due to manufacturing and quality processes. Such problems leads to what if known as xe2x80x9ctiling.xe2x80x9d This is where the brighter apparatus, an array or groups, i.e. of pixels, of a display stand out over the rest of the display. Tiling detracts from the quality of the display and can be an annoyance to a viewer.
What is needed is a color apparatus that can deliver a greater useful viewing angle, have better resolution and contrast, have better uniformity to reduce the possibility of tiling, have the capability to uniformly change the divergence of the light, and that utilizes less expensive LEDs.
The invention is directed to a video display apparatus that is combined to make up a signboard or display. The apparatus has a housing with a planar surface, and a number of pixels arranged in a plane parallel to the housing surface. Each pixel contains a number of LEDs with one or more LEDs in each pixel generating light in a selected color. The LEDs are aligned such that each LED projects light in a direction generally outwardly from the pixel. A holographic optical element receives the light from the LEDs and projects it into a desired pattern.
In an X, Y, Z, coordinate system, the X and Y axis form the XY plane. It is the XY plane that contains the cross section of the pattern of the light. The Z axis is in the direction of propagation of the light from the holographic optical element and is at the center of the cross section of the pattern in the XY plane. The Z axis is perpendicular to the XY plane. The light from the LEDs is emitted from the pixels toward the surface of the housing at a variety of angles. This is referred to as emitting light outwardly from the housing. The light is incident upon a holographic optical element where the holographic optical element, such as a surface relief hologram, is designed to disperse incident light from the LEDs light in a pattern that is symmetrical about the X axis and Y axis. Another case is where the Z axis is substantially perpendicular to the surface of the housing.
The cross section of the transmitted light, from the LED or pixel through the holographic optical element is symmetrical about the X axis and Y axis and is greater in the X direction than the Y direction. The holographic optical element can disperse as well as redirect the light. Light is considered redirected when the difference between the angle between the perpendicular to the surface of the housing and the Z axis is greater than 3 degrees.
Additional optical elements can be used in conjunction with the holographic optical element. The additional elements do not need to be holographic in nature. One such element can be a prismatic sheet that can be used to change the direction of the Z axis of the light dispersed by the holographic optical element.
A reflector can be optionally used to direct light that is not transmitted along the axial line of the LED in a direction more in line with the axial line and outward from the pixel. Each pixel has a number of LEDs. One or more LEDs operate to supply a particular color, which may, but need not, be a primary color. The number of LEDs and the range of colors are factors that can vary depending upon such factors as customer demand. For example, a pixel could be composed of at least one green, blue and red LED. A number of pixel arrays can be combined to form a video image board. The number of pixel arrays that can be combined horizontally and vertically can vary. An LED designed to generate light at a specific color is a color specific LED.
Different holographic optical elements can be interchanged with a video display apparatus. The holographic optical elements can be used to form a variety of shapes including variations of an ellipse. This is referred to as being in a substantially elliptical pattern where the light pattern is greater along the X axis than the Y axis Further, the holographic optical apparatus does not need to be of the form of a surface relief hologram and the holographic optical apparatus can act as a diffuser.
A method for characterizing the output of a video display apparatus in accordance with the invention begins with applying power in pulses, preferably in the form of current at a selected voltage during a predetermined operating time, to the LED or LEDs responsible for a selected color in each pixel. The light output of the LED or LEDs is then measured and recorded. This procedure is repeated for each color responsible LED or LEDs in each pixel. After the light output from each color LED or LEDs is measured and recorded, then the operating time, or pulses of power, of each color LED or LEDs is varied to achieve a desired uniform output for the apparatus. This is referred to as normalizing the apparatus to create uniformity of colors and intensities in the apparatus.
A further step can be introduced such that the initial measured output for each LED is checked to determine if the output is below an acceptable level and if so, the LED apparatus is removed for replacement.